This invention relates to the transmission of television sub-carrier signals, and in particular to the use of such signals for purposes which are not necessarily associated with the broadcast of television signals transmitted via a carrier signal.
It is accepted practice in satellite television transmission to utilise sub-carriers for the transmission of associated television sound signals, and additional sub-carriers for further sound channels or data channels not associated with the television picture; such additional sub-carrier transmissions are generally referred to as "additional services".
Receivers designed to receive additional services transmitted by sub-carriers can have useful application in, for example, the reception of data or facsimile signals, since such receivers can be installed in low cost ground stations having antenna dishes of small aperture (less than 1 meter diameter). Such signals can be broadcast by means of medium power quasi-direct broadcast satellites or high power direct broadcast satellites.
A limit is set, however, on the minimum antenna dish aperture of the receiving ground station by the need to maintain the received carrier to noise (C/N) ratio of the main television carrier signal above the FM threshold for noise-free reception, typically 10 dB. Consequently the aerial dish aperture of the receiving ground station cannot be reduced to the desirable size of less than 1 meter diameter--for example, the current "EUTELSAT" satellites require a ground station receiving antenna of at least 1.5 meter diameter in order to maintain the C/N ratio of above 10 dB.
A known solution to to this problem is to transmit separate signals or "accesses" to the satellite on discrete frequencies which are unrelated to the television signal frequencies. This technique, known as Single Channel Per Carrier (SCPC) suffers, however, from the disadvantage that the frequency-stability of the frequency sources in the transmitter and the receiver has to be very high in order to resolve the weak narrow band signals. In practice this increases the cost of the ground station, even to the point where it is cheaper to employ a larger receiving antenna and revert to sub-carrier operation.